While adhesives are used in various fields in recent years, diversification of the purpose of use requires further improvement in adhesiveness to various types of plastic films, adhesiveness to metal such as copper, aluminum and stainless steel, adhesiveness to glass epoxy, heat resistance, moisture resistance, a sheet life and the like, as compared with the conventionally used adhesives. For example, adhesives for circuit boards including a flexible printed wiring board (which may be hereinafter abbreviated as FPC) are required to have adhesiveness, processability, electrical characteristics, and preservability. For these applications, an epoxy/acrylic butadiene-based adhesive, an epoxy/polyvinyl butyral-based adhesive and the like have been conventionally used.
Particularly, with the recent tendency of the increased density of the wiring and the multilayered flexible printed wiring board, it is strongly required to improve adhesiveness at a high temperature and high humidity. The conventional epoxy/acrylic butadiene-based adhesive and epoxy/polyvinyl butyral-based adhesive were poor particularly in adhesiveness and processability at a high temperature and high humidity, and also insufficient in adhesiveness to metal and plastic films. Also, the stabilized sheet life that can be put into circulation even at ordinary temperatures could not be ensured.
For the above-described problems, a resin composition for an adhesive containing specific polyester polyurethane and an epoxy resin as main components is proposed, for example, in Japanese Patent Laying-Open No. 11-116930 (PTL 1), Japanese Patent Laying-Open No. 2008-205370 (PTL 2), Japanese Patent Laying-Open No. 2007-204715 (PTL 3), and the like. Although it is recognized that the compositions disclosed in PTL 1 can improve the adhesiveness at a high temperature, the adhesiveness at a high temperature and high humidity is still insufficient. According to the composition disclosed in PTL 1, the reaction between the carboxyl group and the epoxy group in the polyurethane resin gradually progresses even at ordinary temperatures, which causes a problem that the sheet life is shortened. Although the composition disclosed in PTL 2 allows improvement in the adhesiveness at a high temperature and high humidity and in the solder resistance to humidification at the time when a plastic film is used as a reinforcing plate, this composition cannot sufficiently improve the solder resistance to humidification at the time when metal is used as a reinforcing plate. Furthermore, the composition disclosed in PTL 2 causes significant decreases in the solder resistance to humidification after storage each at an ordinary temperature and at 40° C. and also in the adhesiveness at a high temperature and high humidity, so that the stabilized sheet life could not be ensured. In PTL 3, the glass transition temperature of the polyurethane resin was raised to successfully improve the adhesiveness and the sheet life at a high temperature. However, raising the glass transition temperature inevitably causes reduction in flexibility of the adhesive layer. Consequently, when the adhesive sheet is cut/punched and when it is peeled off from the release sheet in the manufacturing process of the flexible printed wiring board, the adhesive sheet may be cracked or exfoliated, which leads to a disadvantage that the processing suitability becomes insufficient. Furthermore, the composition disclosed in PTL 3 was insufficient also in adhesiveness at a high temperature and high humidity.
In addition, Japanese Patent Laying-Open No. 2008-019375 (PTL 4) and Japanese Patent Laying-Open No. 2009-084348 (PTL 5) each disclose an adhesive composition into which two types of polyester resins having different glass transition temperatures are blended. According to these adhesive compositions, although the processing suitability at a room temperature can be improved by blending the polyester resin having a relatively high glass transition temperature and the polyester resin having a relatively low glass transition temperature, the flexibility is relatively low on the condition of not more than 5° C. at which the adhesive sheet is generally stored. Thus, satisfactory processability could not be achieved at a low temperature. Furthermore, since the polyester resin is less in cohesive force than the polyurethane resin, the polyester resin is inferior in adhesiveness and heat resistance while the outflow amount is also increased during the pressing process, thereby leading to a disadvantage that the base material is contaminated.